Fluid control structure

ABSTRACT

An optical structure includes a substrate including a cavity on a first surface of the substrate, an optical component on the substrate and an adhesive applied to a side of the optical component to fix the optical component to the substrate. The optical component includes a recess on a second surface of the optical component, the second surface is opposed to the first surface of the substrate, and the recess is provided along an edge of the second surface.

BACKGROUND

Technical Field

The present invention relates to a fluid control structure.

Related Art

Recently, various techniques have been known regarding a fluid controlstructure.

SUMMARY

According to an embodiment of the present invention, there is providedan optical structure. The optical structure includes a substrate, anoptical component and an adhesive. The substrate includes a cavity on afirst surface of the substrate. The optical component is on thesubstrate. The adhesive is applied to a side of the optical component tofix the optical component to the substrate. Furthermore, in the opticalstructure, the optical component includes a recess on a second surfaceof the optical component, the second surface is opposed to the firstsurface of the substrate, and the recess is provided along an edge ofthe second surface.

According to another embodiment of the present invention, there isprovided a fluid control structure. The fluid control structure includesa first component, a second component and a fluid. The first componentincludes a cavity on a first surface of the first component. The secondcomponent is on the first component. The fluid infiltrates into a gapbetween the first component and the second component. Furthermore, inthe fluid control structure, the gap includes a narrow area and a widearea, the wide area has a wider width of the gap than the narrow area,and the wide area is adjacent to the narrow area and closer to thecavity than the narrow area.

According to still another embodiment of the present invention, there isprovided a method for fabricating an optical structure. The methodincludes forming a substrate with a cavity on a first surface of thesubstrate. The method further includes forming an optical component witha recess on a second surface of the optical component. The methodfurther includes mounting the optical component on the substrate. Thesecond surface of the optical component is opposed to the first surfaceof the substrate. The recess is on a periphery of the second surfacewith respect to an opposing area. The opposing area is a part of thesecond surface to be opposed to the cavity. The method further includesapplying an adhesive to a side of the optical component so that theadhesive infiltrates into a gap between the substrate and the opticalcomponent. The method further includes preventing the adhesive fromflowing into the cavity with the recess. The method further includesfixing the optical component to the substrate with the adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a top view of an optical communication system accordingto the first embodiment.

FIG. 2 depicts a side view of a multi-chip module (MCM) according to thefirst embodiment.

FIG. 3A depicts a schematic cross-sectional view of a vertical cavitysurface emitting laser (VCSEL) chip array and a waveguide layeraccording to the first embodiment, taken along the line IIIA-IIIA inFIG. 3C.

FIG. 3B depicts a schematic cross-sectional view taken along the lineIIIB-IIIB in FIG. 3A.

FIG. 3C depicts a bottom view of the VCSEL chip array viewed along thearrow IIIC in FIG. 3A.

FIG. 3D depicts an enlarged bottom view of the VCSEL chip array aroundan emitting area.

FIG. 4A, FIG. 4B, FIG. 4C and FIG. 4D depict a fixing process of theVCSEL chip array to the waveguide layer.

FIG. 5A depicts a simulation result of the flow of the adhesive withouta groove according to a comparative example.

FIG. 5B, FIG. 5C, FIG. 5D and FIG. 5E depict simulation results of theflow of the adhesive with the groove according to the first embodiment.

FIG. 6A, FIG. 6B, FIG. 6C, FIG. 6D and FIG. 6E depict cross-sectionalviews of the VCSEL chip array and the waveguide layer according to othermodifications of the first embodiment.

FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7D, FIG. 7E and FIG. 7F depict bottomviews of the VCSEL chip array according to other modifications of thefirst embodiment.

FIG. 8 depicts a cross-sectional view of a photodiode (PD) chip arrayand the waveguide layer according to the second embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

It is to be noted that the present invention is not limited to theseexemplary embodiments to be given below and may be implemented withvarious modifications within the scope of the present invention. Inaddition, the drawings used herein are for purposes of illustration, andmay not show actual dimensions.

Referring to FIG. 1, there is shown a top view of an opticalcommunication system 1 according to a first embodiment. As shown in FIG.1, the optical communication system 1 may include two multi-chip modules(MCMs) 5. The multi-chip module (MCM) 5 may include a main substrate 10,a central processing unit (CPU) 11, a vertical cavity surface emittinglaser (VCSEL) chip array 12, a laser diode driver (LDD) chip 13, aphotodiode (PD) chip array 14, a trans-impedance amplifiers (TIA) chip15, waveguide layers 161 and 162, and a fiber connector 17. Further, asshown in the figure, the optical communication system 1 may includefiber cables 181 and 182 each having several (e.g., 12 or 24) fibercable cores.

The waveguide layer 161 may include multiple waveguide cores WG whosenumber matches the number of cores of the fiber cable 181 or 182. TheVCSEL chip array 12 may include multiple VCSEL devices whose numbermatches the number of cores (not shown) of the waveguide layer 161. Thewaveguide layer 162 may include multiple waveguide cores WG whose numbermatches the number of cores (not shown) of the fiber cable 181 or 182.The PD chip array 14 may include multiple PD devices whose numbermatches the number of cores of the waveguide layer 162. The VCSEL chiparray 12 is an example of an optical component or an example of a secondcomponent. The waveguide layer 161 is an example of a substrate or anexample of a first component. The MCM 5 is an example of an opticalmulti-chip module.

Referring to FIG. 2, there is shown a side view of the MCM 5 accordingto the first embodiment. As shown in the figure, the waveguide layer 161may be formed on the surface of the main substrate (e.g., an organicsubstrate) 10. The waveguide layer 161 may include the waveguide coresWG and a cladding layer 160. The waveguide layer 161 may be formed as apolymer waveguide. Note that the waveguide layer 161 is an opticalwaveguide-integrated organic structure.

The waveguide layer 161 may be provided with multiple mirror cavities(mirror ablation holes) 165. The mirror cavities 165 are provided at oneend of the waveguide cores WG (left side in FIG. 2) and opened to theVCSEL chip array 12. Each mirror cavity 165 is provided on eachwaveguide core WG. In other words, the number of the mirror cavities 165matches the number of the waveguide cores WG. The mirror cavity 165 isan example of a cavity. Note that the mirror cavities 165 may befabricated by laser ablation instead of dicing blade for high channeldensity.

The waveguide layer 161 may also be provided with multiple mirrorcavities 167. The mirror cavities 167 are provided at the other end ofthe waveguide cores WG (right side in FIG. 2) and opened to the fiberconnector 17. Each mirror cavity 167 is provided on each waveguide coreWG. In other words, the number of the mirror cavities 167 matches thenumber of the waveguide cores WG.

The mirror cavities 165 and 167 may be inclined at an angle of 45° toform reflective surfaces (mirrors M) on boundaries (interfaces) betweenthe waveguide cores WG and the mirror cavities 165 or the mirrorcavities 167. In the embodiment, the boundaries may be provided withouta metal coating, and the mirror cavities 165 and 167 may be filled withair (atmosphere, gas). This configuration enables the mirrors M toreflect the light by total internal reflection (TIR). More specifically,the mirrors M of the mirror cavities 165 reflect the light from theVCSEL chip array 12 to the waveguide cores WG by total internalreflection. The mirrors M of the mirror cavities 167 reflect the lightfrom the waveguide cores WG to the fiber connector 17 by total internalreflection.

FIG. 3A depicts a schematic cross-sectional view of the VCSEL chip array12 and the waveguide layer 161 according to the first embodiment, takenalong the line IIIA-IIIA in FIG. 3C. FIG. 3B depicts a schematiccross-sectional view taken along the line IIIB-IIIB in FIG. 3A. FIG. 3Cdepicts a bottom view of the VCSEL chip array 12 viewed along the arrowIIIC in FIG. 3A. FIG. 3D depicts an enlarged bottom view of the VCSELchip array 12 around an emitting area 121.

In the following explanation, the direction along the axis of thewaveguide core WG is called an axial direction, and the directionperpendicular to the axis of the waveguide core WG is called a widthdirection.

Referring FIGS. 3A to 3D, an explanation is given about theconfiguration around the VCSEL chip array 12 according to the firstembodiment. As shown in FIG. 3A, the VCSEL chip array 12 is provided onthe waveguide layer 161.

As shown in FIGS. 3A to 3C, the VCSEL chip array 12 has a generallyrectangular parallelepiped shape. The VCSEL chip array 12 has a bottomsurface 12A. The VCSEL chip array 12 may include, on the bottom surface12A, emitting areas 121, electrodes 123, 125, connecting areas 127, 129,insulating areas 131, studs 133, and a groove 135. The bottom surface12A is an example of a second surface of the optical component.

The emitting areas 121 are areas emitting the light. The emitting areas121 are arranged in a row corresponding to the row of the mirrors M ofthe mirror cavities 165 (refer to FIGS. 3C and 3D). The VCSEL chip array12 is positioned on the waveguide layer 161 so that each emitting area121 is aligned with the corresponding mirror M provided on the waveguidecore WG. This configuration allows the light emitted from the emittingarea 121 to be reflected at the mirror M. The light reflected at themirror M passes through the waveguide core WG. In the shown example, theemitting area 121 has a generally round shape in a plane view.

The electrodes 123 are electrically connected with the respectiveemitting areas 121 via the connecting areas 127. The electrodes 125 arealso electrically connected with the respective emitting areas 121 viathe connecting areas 129. The electrodes 123, 125 are arranged in a rowalong the row of the mirrors M. In the shown example, the electrodes123, 125 have a generally round shape in a plane view. In the followingexplanation, a set of the emitting area 121, the electrodes 123, 125 andthe connecting areas 127, 129 may be called an emitting circuit 120.

As shown in FIG. 3C, the insulating areas 131 are provided on a centralarea of the bottom surface I 2A. The insulating areas 131 are arrangedin a row along the longitudinal direction of the VCSEL chip array 12.Each insulating area 131 has a generally square shape in a plane view.The insulating area 131 surrounds an area where the emitting circuit 120is provided. The insulating area 131 may be a resin, such as polyimide.

The studs 133 are respectively provided on the electrodes 123, 125. Thestuds 133 are electrical contact points for connection between theelectrodes 123, 125 and the main substrate 10. The studs 133 may be ametal, such as Au. Each stud 133 is a protruded portion on the bottomsurface 12A. More specifically, the stud 133 is the most protrudedportion of the he VCSEL chip array 12 toward the main substrate 10. Thestud 133 is an example of a protrusion on the second surface.

The groove 135 is a recessed portion on the bottom surface 12A. Thedetailed configuration of the groove 135 will be described later.

The waveguide layer 161 has an upper surface 161A. The waveguide layer161 may include the mirror cavities 165 on the upper surface 161A. Theupper surface 161A is an example of a first surface of the substrate.The waveguide layer 161 may also include through holes (holes) 169penetrating the waveguide layer 161. Each through hole (hole) 169 isprovided on a position corresponding to the stud 133 provided on theelectrodes 123, 125. The inner space of the through hole 169 is largerthan the stud 133. This configuration enables at least a part of thestud 133 to be disposed in the corresponding through hole 169. In theshown example, the through hole 169 has a generally round shape in aplane view.

As shown in FIGS. 3A and 3B, the VCSEL chip array 12 is glued onto thewaveguide layer 161 using an adhesive 210. The adhesive 210 may be aphotocurable material, such as ultraviolet (UV) curing resin or athermal curing material. The adhesive 210 is an example of a fluid.

The VCSEL chip array 12 is provided on the waveguide layer 161 with thestud 133 in the through hole 169. In other words, the stud 133 ispositioned inside the waveguide layer 161. That is to say, a protrudedpart of the VCSEL chip array 12 is inserted in the through hole 169.

The stud 133 is fixed with solder 110 within the through hole 169. Thisconfiguration enables the emitting circuit 120 (the VCSEL chip array 12)to be electrically connected with the main substrate 10. Note that ajoint between the stud 133 and the solder 110 (the main substrate 10)may be brittle depending on the bonding strength of solder bonding.

To arrange the joint between the stud 133 and the solder 110 in thethrough hole 169 can shorten a gap GP between the bottom surface 12A ofthe VCSEL chip array 12 and the upper surface 161A of the waveguidelayer 161. Since the light from the emitting areas 121 passes throughthe gap GP, to shorten the gap GP can decrease an optical loss of thelight during passing through the gap GP.

The gap GP between the bottom surface 12A and the upper surface 161A,more specifically the gap GP between the bottom surface 12A on an edgeportion of the VCSEL chip array 12 and the upper surface 161A of thewaveguide layer 161, as denoted by L0 in FIG. 3A, may be less than 20μm, preferably less than 10 μm, and more preferably less than 5 μm.Further, the gap GP may be 0 μm. In other words, the VCSEL chip array 12may contact the upper surface 161A.

Here, in the manufacturing process of the optical communication system 1(refer to FIG. 1), the VCSEL chip array 12, the waveguide layer 161 andthe main substrate 10 may be heated, e.g. in a reflow process or in asoldering process. Due to coefficient of thermal expansion (CTE)mismatch between the VCSEL chip array 12 and the main substrate 10 (thewaveguide layer 161), the joint between the stud 133 and the solder 110may be broken under high temperature condition. The damage of the jointbetween the stud 133 and the solder 110 may be more severe on both sidesof the VCSEL chip array 12 in the longitudinal direction than a centralarea of the VCSEL chip array 12.

As mentioned above, the VCSEL chip array 12 is glued onto the waveguidelayer 161 using the adhesive 210. The adhesive 210 suppresses a changeof a relative position between the VCSEL chip array 12 and the waveguidelayer 161, so that the adhesive 210 decreases stress on the jointbetween the stud 133 and the solder 110.

The adhesive 210 is provided on the periphery of the VCSEL chip array12. In other words, the adhesive 210 is applied as a sidefill structure(sidefill sealing). Sealing by sidefill refers to the adhesive 210 beingapplied to at least a side (periphery) of the first electrical component(e.g. the VCSEL chip array 12) mounted on the second electricalcomponent (e.g. the waveguide layer 161). In the sealing by sidefill,the adhesive 210 may not be applied to a central portion of an opposingarea of the VCSEL chip array 12 and the waveguide layer 161 in a planeview. In other words, the adhesive 210 does not get to the centralportion of the opposing area (under the VCSEL chip array 12).

FIGS. 4A to 4D depict a fixing process of the VCSEL chip array 12 to thewaveguide layer 161.

Referring FIGS. 4A to 4D, an explanation is given about the generalmethod for fixing the VCSEL chip array 12 to the waveguide layer 161 inthe first embodiment.

As shown in FIG. 4A, the VCSEL chip array 12 is first mounted on thewaveguide layer 161. The stud 133 of the VCSEL chip array 12 is insertedin the through hole 169 of the waveguide layer 161 to be bonded to themain substrate 10 with the solder 110.

As shown in FIG. 4B, the adhesive 210 in a fluidized state is applied(injected) around the VCSEL chip array 12.

As shown in FIG. 4C, a part of the adhesive 210 distributes through thegap GP due to capillary effect (refer to arrows A0).

As shown in FIG. 4D, curing light is applied to cure the adhesive 210.This enables to fix the VCSEL chip array 12 to the waveguide layer 161.Instead of the photocurable material, the adhesive 210 may be athermally cured sidefill adhesive.

In the above mentioned process, the adhesive 210, i.e. viscous sidefillmaterial, is solidified on the whole periphery of the VCSEL chip array12 except the center of the bottom surface 12A.

Returning to FIGS. 3A, 3B and 3C, an explanation is given about thegroove 135 according to the first embodiment. As shown in FIG. 3C, thegroove 135 is provided on the bottom surface 12A along the edges of theVCSEL chip array 12. In other words, the groove 135 is provided on theouter periphery of the bottom surface 12A (in proximity to the edges ofthe VCSEL chip array 12). The groove 135 is an example of a recess. Thegroove 135 is an example of a wide area. The outer periphery of thegroove 135 is an example of a narrow area.

The groove 135 may be fabricated by laser ablation or reactive ionetching (RIE) in the manufacturing process of the VCSEL chip array 12.

The groove 135 controls a flow of the adhesive 210 in the fluidizedstate. More specifically, the groove 135 controls a capillary phenomenonof the adhesive 210. The groove 135 prevents the adhesive 210 fromflowing into the mirror cavities 165 by enlarging the height of the gapGP.

Control the flow of the adhesive 210 may influence a yield in themanufacturing process of the optical communication system 1 (refer toFIG. 1). Thus, use of the groove 135 enables improvement of an assemblyyield of the VCSEL chip array 12.

Here, if the distributing adhesive 210 flows (infiltrates) into themirror cavities 165, the mirror cavities 165 may not function as themirrors M reflecting the light by total internal reflection. In otherwords, the reflectivity at the mirrors M may be declined.

Further, if sealing by underfill is applied instead of the sealing bysidefill, the adhesive 210 may get to the central portion of theopposing area. That is to say, the sealing by underfill may result inthe above undesired flow of the adhesive 210 into the mirror cavities165. Note that the sealing by sidefill according to the first embodimentneeds a small amount of the adhesive 210 compared to the sealing byunderfill.

The groove 135 is provided on the bottom surface 12A so as to surroundan area where the mirror cavities 165 are provided, more specificallywhere the mirror cavities 165, the through holes 169, and the emittingareas 121 are provided.

This configuration enables prevention of the adhesive 210 from flowinginto the through holes 169. Note that since the through holes 169 andthe mirror cavities 165 are provided adjacent to each other, the flow ofthe adhesive 210 into the through holes 169 may result in the adhesiveflowing into the mirror cavities 165. In other words, prevention of theadhesive 210 from flowing into the through holes 169 may contribute tothe prevention of the adhesive 210 from flowing into the mirror cavities165.

The above mentioned configuration also enables prevention of theadhesive 210 from flowing into the light path of the light from theemitting areas 121. In other words, the light from the emitting areas121 does not traverse the adhesive 210. This configuration reducesoptical loss by the light traversing the adhesive 210. Thisconfiguration also increases options of a material for the adhesive 210since the adhesive 210 is not required to be transparent to the lightfrom the emitting areas 121.

As shown if FIG. 3C, the groove 135 may include multiple liner grooveportions, i.e. the first portion 135A, the second portion 135B, thethird portion 135C and the fourth portion 135D. The first portion 135Ato the fourth portion 135D are provided along the respective edges ofthe VCSEL chip array 12. The first portion 135A and the second portion135B are provided on both sides along the axial direction. The thirdportion 135C and the fourth portion 135D are provided on both sidesalong the width direction.

In one example, the height of the VCSEL chip array 12 (refer to L1 inFIG. 3A) may be 150 μm, the length of the VCSEL chip array 12 (refer toL2 in FIG. 3C) may be 300 μm, and the width of the VCSEL chip array 12(refer to L3 in FIG. 3C) may be 1 mm. In this example, the bottomsurface 12A has a fringe area 12B which is not covered by the insulatingareas 131. The width of the fringe area 12B (refer to L4 in FIG. 3C) maybe equal to or more than 30 μm.

The groove 135 is provided in the fringe area 12B. The width of thegroove 135 (refer to L5 in FIG. 3C) may be 5 μm to 100 μm, preferably 10μm to 50 μm. Note that the small width of the groove 135 can reduce thesize of the VCSEL chip array 12. The depth of the groove 135 (refer toL6 in FIG. 3A) may be 10 μm to 100 μm, preferably 20 μm to 50 μm. Notethat the depth of the groove 135 is larger than the height of the gap GP(refer to L0 in FIG. 3A), and preferably twice as much or more than theheight of the gap GP.

The groove 135 enlarges the height (size) of the gap GP such that thesize of the gap GP increases abruptly in the groove 135. In the shownexample, the cross-sectional shape of the groove 135 is rectangular(refer to FIG. 3A).

FIG. 5A depicts a simulation result of the flow of the adhesive 210without the groove 135 according to a comparative example. FIGS. 5B to5E respectively depict simulation results of the flow of the adhesive210 with the groove 135 according to the first embodiment.

An explanation is given about the simulation of fluid dynamics of theadhesive 210 in the gap GP. The simulation of fluid dynamics isconducted in a two dimensional flow.

As shown in FIG. 5A, the general configuration for the simulationincludes the VCSEL chip array 12 and the waveguide layer 161. The VCSELchip array 12 and the waveguide layer 161 are provided in theatmosphere. In the initial state, the adhesive 210 in a fluidized stateis enclosed in fields on a side (left side in FIG. 2) of the VCSEL chiparray 12.

The properties of the adhesive 210 in a fluidized state are as follows:the surface tension is 0.03 kg/s², the kinetic viscosity is 0.1 m²/s,the contact angle is 24 degrees. The gap GP (refer to L0 in FIGS. 5A and5B) is 1 μm in FIGS. 5A and 5B, 5 μm in FIG. 5C, 10 μm in FIG. 5D, and20 μm in FIG. 5E. The simulation duration is 90 s.

As shown in FIG. 5A, the simulation results without the groove 135indicate that the adhesive 210 infiltrates (flows) into the gap GP(refer to the arrow Al). The infiltrating length in the axial directionis indicated by L11. On the other hand, as shown in FIG. 5B, thesimulation results with the groove 135 indicate less infiltration of theadhesive 210 as indicated by the infiltrating length L12, which isshorter than the infiltrating length L11 in FIG. 5A. Further, theadhesive 210 stops at the groove 135 (in front of the groove 135).According to this simulation, providing the groove 135 on the VCSEL chiparray 12 may control the flow of the adhesive 210. In other words, thegroove 135 suppresses the volume of the adhesive 210 infiltrating intothe gap GP.

As shown in FIGS. 5B to 5E, the infiltrating length L12 in FIG. 5B, theinfiltrating length L13 in FIG. 5C, the infiltrating length L14 in FIG.5D, and the infiltrating length L15 in FIG. 5E become longer in thisorder. According to this simulation, adjusting the size of the gap GPmay control the flow of the adhesive 210. For example, the simulationresults indicate that a smaller gap GP equates with a shorterinfiltrating length. Further, the adhesive 210 may be stopped by thegroove 135 more effectively if the height of the gap GP is equal to orless than 5 μm.

FIGS. 6A to 6E respectively depict cross-sectional views of the VCSELchip array 12 and the waveguide layer 161 according to othermodifications of the first embodiment.

As shown in FIG. 6A, the waveguide layer 161 may include a groove 162 onthe upper surface 16I A. The groove 162 may be provided along the edgesof the VCSEL chip array 12. In other words, the groove 162 is opposed tothe groove 135 of the VCSEL chip array 12, and may have a correspondingshape to the groove 135. The combination of the groove 162 and thegroove 135 suppresses the flow of the adhesive 210 more effectively.

In this modification, the thickness from the bottom of the groove 162 tothe waveguide core WG (refer to L21 in FIG. 6A) may be equal to or morethan 10 μm to prevent the light from running out from the waveguide coreWG. Note that the groove 162 of the waveguide layer 161 may be providedinstead of the groove 135 of the VCSEL chip array 12. In other words,the groove 135 can be replaced with the groove 162 of the waveguidelayer 161.

As shown in FIG. 6B, the waveguide layer 161 may include a bank 164instead of the groove 135 of the VCSEL chip array 12. The bank 164 maybe provided along the edges of the VCSEL chip array 12. The bank 164suppresses the flow of the adhesive 210. A material of the bank 164 maybe dispensed and cured. For example, the bank 164 may be provided bysilk screen with a polymeric material. In this modification, the bank164 has a hemisphere shape. In other words, the top surface of the bank164 has a curved surface and protrudes towards the VCSEL chip array 12.The diameter of the bank 164 (refer to L22 in FIG. 6B) may beapproximately 30 μm, and the height of the bank 164 (refer to L23 inFIG. 6B) may be approximately 20 μm.

As shown in FIG. 6C, the waveguide layer 161 may include a hydrophobicarea 166 on the upper surface 161A. The hydrophobic area 166 is atreated surface having poorer wettability to a liquid adhesive thanother areas surrounding the treated surface. The hydrophobic area 166 isprovided along the edges of the VCSEL chip array 12. In other words, thehydrophobic area 166 is opposed to the groove 135 of the VCSEL chiparray 12. The hydrophobic area 166 suppresses the flow of the adhesive210 with the groove 135.

As shown in FIG. 6D, the VCSEL chip array 12 may include a bank 136instead of the groove 135 (refer to FIG. 3A). In other words, the groove135 can be replaced with the bank 136 on the VCSEL chip array 12. Thebank 136 suppresses the flow of the adhesive 210. The bank 136 may beprovided in the manufacturing of the VCSEL chip array 12 with a metalmaterial or an insulation material. For example, the bank 136 may beprovided by lithography process in the manufacturing of the VCSEL chiparray 12. The width of the bank 136 (refer to L24 in FIG. 6D) may beequal to or more than 10 μm, and the height of the bank 136 (refer toL25 in FIG. 6D) may be equal to or more than 10 μm.

As shown in FIG. 6E, the VCSEL chip array 12 may include multiplegrooves, i.e. an outer groove 137 and an inner groove 138. The outergroove 137 is provided along the edges of the VCSEL chip array 12, andthe inner groove 138 is provided inside of and along the outer groove137. In this modification, use of the two grooves 137, 138 prevents theadhesive from infiltrating into the mirror cavities 165 effectively.

FIGS. 7A to 7F respectively depict bottom views of the VCSEL chip array12 according to other modifications of the first embodiment. The shapeof the groove 135 provided on the VCSEL chip array 12 may not be limitedto the shape as shown in FIG. 3C.

As shown in FIGS. 7A to 7C, the groove 135 may have a discontinuousarea. In other words, the shape of the groove 135 is not limited to onethat completely surrounds the central area of the bottom surface 12A. Asshown in FIG. 7A, the groove 135 may include the first portion 135A andthe second portion 135B, eliminating the third portion 135C and thefourth portion 135D (refer to FIG. 3C). In other words, the groove 135shown in FIG. 7A is provided on both sides along the axial direction. Asshown in FIG. 7B, the groove 135 may include the first portion 135A, thesecond portion 135B, and the third portion 135C, eliminating the fourthportion 135D (refer to FIG. 3C). As shown in FIG. 7C, the groove 135 mayinclude the first portion 135A to the fourth portion 135D with the thirdportion 135C and the fourth portion 135D each including a discontinuouspart.

As shown in FIG. 7D, the groove 135 may have an uneven width. Forexample, the width of the first portion 135A and the second portion 135Bmay be wider than the width of the third portion 135C and the fourthportion 135D.

As shown in FIG. 7E, the groove 135 may include a curved portion. Forexample, the groove 135 shown in FIG. 7E has a generally rectangle shapewith rounded corners.

As shown in FIG. 7F, the groove 135 may be provided on each insulatingarea 131 instead of the fringe area 12B (refer to FIG. 3C). For example,grooves 135E to 135H may be provided on a periphery of the respectiveemitting circuits 120.

Here, the cross-sectional shape of the groove 135 may not be limited tothe rectangular shape as shown in FIG. 3A as long as the groove 135enlarges the height of the gap GP. The cross-sectional shape of thegroove 135 may be a trapezoid shape, a curved shape, or a V-shape. Thegroove 135 may be provided with an uneven bottom.

FIG. 8 depicts a cross-sectional view of the PD chip array 14 and thewaveguide layer 162 according to the second embodiment. Theabove-mentioned configurations may also be applicable to any opticalcomponent (optoelectronic chip), such as the PD chip array 14. Forexample, as shown in FIG. 8, the PD chip array 14 may include a groove145 on a bottom surface 14A along edges of the PD chip array 14. Thegroove 145 may prevent the adhesive 210 from flowing into the mirrorcavities 165. In other words, the groove 145 may control the flow of theadhesive 210.

What is claimed is:
 1. An optical structure, comprising: a substrateincluding a cavity and a hydrophobic area on a first surface of thesubstrate; an elongated optical component on the substrate including arecess along an edge of a second surface of the elongated opticalcomponent, the second surface being opposed to the first surface of thesubstrate and the recess being opposed to the hydrophobic area; and anadhesive applied along the elongated optical component in a longitudinaldirection to fix the elongated optical component to the substrate,wherein the cavity is free of the adhesive responsive to the flowprevention of the recess and the hydrophobic area.
 2. The opticalstructure of claim 1, wherein the recess surrounds an area on the secondsurface, and the area is a part of the second surface to be opposed tothe cavity.
 3. The optical structure of claim 1, wherein the substrateincludes a hole, the elongated optical component includes a protrusionon the second surface, and at least a portion of the protrusion isinserted in the hole.
 4. The optical structure of claim 3, furthercomprising a main substrate, the substrate being provided on the mainsubstrate, wherein the elongated optical component is electricallyconnected with the main substrate via the protrusion in the hole of thesubstrate.
 5. The optical structure of claim 4, wherein the adhesiveinfiltrates into a gap between the substrate and the elongated opticalcomponent due to capillary effect, and the recess prevents the adhesivefrom flowing into the hole.
 6. The optical structure of claim 1, whereinthe substrate includes a waveguide core and the cavity intersects thewaveguide core, and an interface between the waveguide core and thecavity reflects a light passing through the waveguide core.
 7. Theoptical structure of claim 6, wherein the cavity is filled with gas, andthe light is reflected at the interface by total internal reflection. 8.The optical structure of claim 7, wherein the adhesive is located apartfrom a light path of light traveling between the elongated opticalcomponent and the interface.
 9. The optical structure of claim 1,wherein the adhesive infiltrates into a gap between the substrate andthe elongated optical component due to capillary effect, and the recessprevents the adhesive from flowing into the cavity.
 10. A method forfabricating an optical structure, the method comprising: forming asubstrate with a cavity and a hydrophobic area on a first surface of thesubstrate; forming an elongated optical component with a recess on asecond surface of the elongated optical component; mounting theelongated optical component on the substrate, the second surface of theelongated optical component being opposed to the first surface of thesubstrate, the recess being on a periphery of the second surface withrespect to an opposing area, the recess being opposed to the hydrophobicarea, the opposing area being a part of the second surface to be opposedto the cavity; applying an adhesive along the elongated opticalcomponent in a longitudinal direction so that the adhesive infiltratesinto a gap between the substrate and the elongated optical component;preventing the adhesive from flowing into the cavity using the recessand the hydrophobic area; and fixing the elongated optical component tothe substrate with the adhesive.
 11. The method of claim 10, furtherincludes inserting at least a portion of a protrusion on the secondsurface of the elongated optical component into a hole included in thesubstrate.
 12. The method of claim 11, further includes electricallyconnecting the elongated optical component to a main substrate on thesubstrate via the protrusion in the hole of the substrate.
 13. Themethod of claim 12, wherein the adhesive infiltrates into a gap betweenthe substrate and the elongated optical component due to capillaryeffect, and the recess prevents the adhesive from flowing into the hole.14. The method of claim 10, further includes filling the cavity with agas.
 15. The method of claim 10, wherein the adhesive infiltrates into agap between the substrate and the elongated optical component due tocapillary effect, and the recess prevents the adhesive from flowing intothe cavity.